Protection device for load circuits

ABSTRACT

A protection device protecting a load circuit by shutting off a power thereof depending on an estimated present temperature of a wire provided in the load circuit to connect a load. The protection device has a first temperature estimation device estimating a rising temperature of a conductor including the wire based on both a current in the conductor and thermal properties of the conductor, a second temperature estimation device estimating a falling temperature of the conductor based on the thermal properties, a third temperature estimation device estimating an arc-induced rising temperature of the conductor when arcing occurs in the conductor, and a fourth temperature estimation device estimating the present temperature of the conductor deduced from the above estimated temperatures. If the estimated temperature exceeds a predetermined allowable temperature, the protection device shut off the power of the load circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a protection circuit for protectinga load circuit having a power supply, a switch, and a load. Theprotection device detects an increase of temperature of a conductivewire used therein.

2. Description of the Related Art

A conventional load circuit supplying power to a load such as a bulb, amotor or the like, has a battery and an electric switch (semiconductorswitch, etc.) provided between the battery and the load. Thesecomponents are connected by conductive wires. The load circuit furtherhas a control device to turn on/off the electric switch. Specifically,the control device outputs a drive signal or a stop signal to theelectric switch so that the load is driven or stopped.

In the load circuit as described above, a protection function providedby a fuse or the like so as to shut down the power immediately when acurrent flowing in the load exceeds a predetermined threshold value ofthe current, so that damage to the load, the wire, the electric switchand the like is avoided.

The protection function as described above can protect the load circuititself only in a dead short circuit, that is, when the current hasapparently exceeds the threshold value. However, it cannot do so in arare short circuit, which is a state where the current is larger than anormal value, but does not exceed the threshold value.

Under such a rare short circuit condition, temperature of the conductivewire would rise due to Joule heat generated therein. If a heating rateof the wire exceeds a cooling rate thereof, problems such as smokeemission from the wire or burnout of the wire may occur.

To solve such a problem, a protection apparatus is known in JapanesePatent Laid-Open Publication 2002-084654. According to this device, whena current flows in a load, the device calculates Joule heat based on themeasured current. When the current does not flow, it calculates radiatedheat, and further it calculates heat generated by arcing which occursjust after a power supply has been shut down. If the total heatcalculated from the sum of the radiated heat and the generated heatexceeds a predetermined value, the device would shut down a circuitsupplying power to the load.

However, the protection apparatus for the load circuit as disclosedabove determines whether the load circuit is shut down or not dependingon cumulative heat of generated and radiated heat and it does not takeinto account an effective rate of increase of temperature. That is, if athick wire was used and the generated heat therefrom was large,temperature of the wire would not rise very much because the heatradiated from the wire sufficiently exceeds the heat generated therein.Consequently, a problem would occur, in which the circuit would beforcibly shut down irrespective of the fact that power can still beapplied to the load device.

On the contrary, if a thin wire was used and the amount of generatedheat was small, temperature of the wire would unexpectedly rise but thecircuit would not be shut down irrespective of substantial smokeemission from the wire and burnout thereof.

SUMMARY OF THE INVENTION

In light of the above-described problems, an objective of the presentinvention is to provide a protection device for a load circuit, whichdetermines shutdown of the load circuit depending on the temperature ofthe wire connecting to a load when the above-described rare shot circuitsituation occurs.

An aspect of the present invention is to provide a protection device fora load circuit having a power supply, a switch and a load, comprising acurrent detection device detecting and measuring a current in aconductor connected between the power supply and the load; a firsttemperature estimation device estimating an increasing temperature ofthe conductor, when the current is detected by a current detectiondevice; a second temperature estimation device estimating a decreasingtemperature of the conductor when the current detection device does notdetect the current or detects that the current is being decreased, athird temperature estimation device estimating an arc-induced increaseof temperature of the conductor caused by arcing which occurs just afterthe current has returned to a normal current; a fourth temperatureestimation device estimating a present temperature of the conductorbased on the estimated increasing, decreasing, and arc-induced increasetemperatures; a temperature determination device determining whether thepresent temperature of the conductor estimated by the fourth temperatureestimation device exceeds a predetermined threshold temperature; and acontrol device that shuts off power to the load circuit when thetemperature determination device determines that the present temperatureof the conductor exceeds the predetermined threshold temperature,wherein the increasing temperature is estimated base on both the currentmeasured by the current detection device and thermal properties of theconductor; the decreasing temperature of the conductor estimated basedon the thermal properties of the conductor; and the arc-induced increaseof temperature is estimated based on the current just before the currenthas returns to normal current.

Further, the thermal properties of the conductor which is used forestimation of the increasing and decreasing temperatures may be athermal resistivity R thereof and a heat capacity C thereof.

According to the above configuration, when the current measured by thecurrent detection device would increase or be constant, therising/increasing temperature of the conductor, including a wire and acontact conductor, can be estimated based on both the measured currentand the thermal properties of the conductor. When the current detectiondevice does not detect the current or detect the current decreases, thefalling/decreasing temperature of the wire can be estimated based on thethermal properties of the conductor. Further, the arc-inducedrising/increasing temperature is induced by the arcing that occurs whenthe current increases and then returns to the normal current.Furthermore, the present temperature of the wire can be estimated by useof the three estimated temperatures described above. If the presenttemperature of the wire exceeds the predetermined threshold value, theprotection circuit determines that the wire could be burned out. If so,it will shut down the power of the load circuit. Accordingly, since thisprotection circuit's determination is derived from the estimated presenttemperature of the wire, the shutdown of the load circuit can beaccurately performed.

Further, since the thermal resistance R and the heat capacity C of theconductor including the wire, which vary depending on the conductor, isused for the thermal properties for the above estimations, these fourtemperatures can be accurately estimated, so that it is possible toprecisely control the shutdown of the load circuit.

Furthermore, since the estimations of the rising/increasing and thefalling/decreasing temperatures of the wire may be estimated based onformulas which show heat generation and heat radiation of the conductorwhile taking into account the thermal resistance and the heat capacitythereof, the four temperatures can be accurately estimated, so that itis possible to more precisely control the shutdown of the load circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of a load circuit in which anembodiment of a protection device is used.

FIG. 2 is a functional block diagram of a control circuit shown in FIG.1

FIG. 3 is a flowchart showing processes performed in the protectiondevice.

FIG. 4A is a chart indicating variation of a temperature of a wire fromin transit time from when a current starts flowing or is increasing inthe wire; and FIG. 4B is a chart indicating variation of the same whenthe current becomes zero or decreases after the temperature issaturated.

FIG. 5A is a chart indicating variation of the temperature of the wirefrom in transit time from when a current starts flowing or is increasingin the wire; and FIG. 5B is a chart indicating variation of the samewhen the current decreases before the temperature is saturated.

FIG. 6 is an arc-related map stored in a third temperature estimationdevice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be explained hereinafterwith reference to the drawings. FIG. 1 is a circuit diagram of a loadcircuit in which a protection circuit is used. The load circuit may beused in a vehicle so that a battery provided in the vehicle to suppliespower to a load, such as a bulb, a motor or the like.

As shown in the same figure, the load circuit 1 is provided between abattery 2 in a vehicle and a load 4, such as a bulb, a motor and thelike. The load circuit 1 has an electrical switch (switch) 3 such as aMOSFET to supply power from the battery 2 to the load 4.

The load circuit 1 further has an ammeter (a current detection device) 5for detecting and measuring a current flowing to the load 4, and acontrol circuit 6 for controlling ON and OFF states of the electricalswitch 3. Here, the battery 2 is electrically connected to theelectrical switch 3 by a wire 7. In the same way, the electrical switch3 is electrically connected to the load 4 by the wire 7. Accordingly, inthis embodiment, a protection circuit 10 for the load circuit includesthe electrical switch 3, the ammeter 5 and the control circuit 6.

FIG. 2 is a functional block diagram showing a detailed configuration ofthe control circuit 6. As seen in this figure, the control circuit 6comprises a rising/increasing temperature estimation device (a firsttemperature estimation device) 61, a falling/decreasing temperatureestimation device (a second temperature estimation device) 62, anarc-induced rising/increasing temperature estimation device (a thirdtemperature estimation device) 63, a present temperature estimationdevice (a fourth temperature estimation device) 64, a temperaturedetermination device 65, and a switch control device (a shutoff controldevice) 66.

The rising/increasing temperature estimation device 61 estimates atemperature increase of the wire 7 at a predetermined sampling rate(e.g. 5 msec). The estimation is based on both a value of the currentflowing to the load 4, as measured by the ammeter 5 when the electricalswitch is turned ON, and predetermined thermal properties (a thermalresistivity R and a thermal capacity C, as described below) of aconductor including the wire 7 and a contact conductor.

The falling/decreasing temperature estimation device 62 estimates adecrease of temperature of the wire 7 in the predetermined samplingrate. The estimation is based on both an estimated present temperatureof the wire 7 and the predetermined thermal properties. Here, theestimated present temperature is determined when the ammeter 5 does notdetect the current due to a disconnection between the wire 7 and thecontact conductor in the circuit, when the ammeter 5 cannot detect thecurrent after the current is decreased, or when the current starts todecrease.

The temperature of the wire 7 is occasionally increased by arcing whichoccurs just after the normal current in the load has recovered followinga connection between the wire 7 and the contact conductor. Thearc-induced temperature estimation device 63 estimates an arc-inducedrising/increasing temperature of the wire 7 that is induced by thearcing, based on the current measured just before the normal current hasbeen recovered.

The present temperature estimation device 64 estimates a presenttemperature of the wire 7 by estimation that is based on temperaturesestimated by the rising/increasing temperature estimation device 61, thefalling/decreasing temperature estimation device 62, and the arc-inducedrising/increasing temperature estimation device 63. The presenttemperature estimation device 64 includes a memory 64 a to store theestimated present temperature.

The temperature determination device 65 compares the present temperatureTnow estimated by the present temperature estimation device 64 with apredetermined maximum allowable temperature (a predetermined thresholdtemperature), Tth. If the temperature determination device 65 determinesthat Tnow is higher than Tth, the device 65 outputs a circuit-shutoffsignal to the switch control device 66.

The switch control device (shutoff control device) 66 receives thecircuit-shutoff signal and then turns off the electrical switch 3 so asto stop supplying power and protect a circuit.

The functional configuration of the control circuit 6 as described aboveis related only to a configuration applied when the so-called rare-shortcircuit occurs in the load circuit, and description of the dead-shortcircuit configuration is omitted.

Next, estimation methods of the rising/increasing or falling/decreasingtemperature by the first, second and third temperature estimationdevices are explained below.

(A) Estimation of the Rising/Increasing Temperature by the TemperatureEstimation Device 61

The temperature of the wire 7 increases by the current flowing therein,this temperature is determined from the formula (1) as follows;

$\begin{matrix}{T_{1} = {T_{2} + {i^{2}{r \cdot R \cdot \left( {1 - ^{{- \frac{1}{C \cdot R}} \cdot t}} \right)}}}} & (1)\end{matrix}$

,wherein T1 is a temperature of the wire [° C.]; T2 is an ambienttemperature [° C.]; i is a current [A]; r is an electric resistance inthe conductor [Ω]; R is a thermal resistance of the conductor [° C./W];C is a heat capacity of the conductor [J/° C. or W·sec/° C.]; and t istransit time [sec].

In the above formula, the ambient temperature T2 is an initialtemperature of the wire 7 when the electrical switch 3 is turn on. Forexample, T2 may be 25° C. in normal circumstances and it may be 85° C.in an engine compartment where an engine in a vehicle is running. Thecurrent i is determined by a measurement obtained from the ammeter 5.The electrical resistance r is a resistance of the conductor includingthe wire 7, and is constant. The thermal resistance R is a valueindicating the conductor's ability to conduct heat. The thermalresistance R depends on intrinsic properties such as the conductor'smaterial, thickness, and shape. The heat capacity C represents an amountof heat which is required to increase the temperature of the conductorby 1° C. The heat capacity also depends on the intrinsic properties ofthe conductor.

Consequently, if the current i, and transit time t, are determined thepresent temperature is estimated by the formula (1).

(B) Estimation of the Falling/Decreasing Temperature by the TemperatureEstimation Device 62

If the ammeter 5 does not detect a current or detects that the currentdecreases, the temperature T1 is determined by taking into account heatradiated from the conductor by a formula (2) as follows;

$\begin{matrix}{T_{1} = {T_{2} + {i^{2}{r \cdot R \cdot ^{{- \frac{1}{C \cdot R}} \cdot t}}}}} & (2)\end{matrix}$

, wherein T1 is the temperature of the wire [° C.]; T2 is the ambienttemperature [° C.]; i is the current [A]; r is the electrical resistancein the conductor [Ω]; R is the thermal resistance of the conductor [°C./W]; C is the heat capacity of the conductor [J/° C. or W·sec/° C.]; tis the transit time [sec].

In the formula (2), the ambient temperature T2 is the temperature of thewire 7 determined when the ammeter 5 does not detect the current ordetects that the current is decreasing. If the temperature of the wire 7is not saturated, the current i in the formula (2) is defined as acurrent which would be flowing at this temperature which is assumed tobe a saturated temperature in the formula (1). If the temperature of thewire 7 is saturated when the ammeter 5 does not detect the current ordetects that the current is decreased, the current i in the formula (2)is defined as a current which is measured just before the ammeter 5 doesnot detect the current or detects that the current has decreased.Consequently, if the current i and transit time, t, are determined asdescribed above, the present temperature of the wire 7 is estimated bythe formula (2).

(C) Estimation of Arc-Induced Rising/Increasing Temperature by theArc-Induced Temperature Estimation Device 63

The memory 64 a preliminarily stores data as a arc-related map shown inFIG. 6, which includes a temporal high current i which flows just beforethe normal current is recovered following connection of the wire 7 withthe contact conductor, and a temperature increment Q(i) caused by thetemporal high current. Consequently, when the temporal high current i ismeasured before the normal current is recovered following connection ofthe wire 7 with the contact conductor, the arc-induced temperatureestimation device 63 finds the temperature increment Q(i) from the data,based on the measured temporal high current i, and estimates thearc-induced rising/increasing temperature of the wire 7 based on thefollowing formula (3);

T1=T2+Q(i)  (3),

where the ambient temperature T2 is determined when the normal currentis recovered following connection of the wire 7 with the contactconductor.

Next, an operation of the load circuit 1, according to the embodimentdescribed above, is explained hereinafter with reference to theflowchart shown in FIG. 3. Here, each process shown in this chart iscarried out periodically in a predetermined sampling rate (e.g. 5 msec).

First, in step S1, the ammeter 5 determines if a load current is flowingto the load 4. In other words, it is determined if the load current isflowing after the electrical switch 3 is turned on and an electricalconnection between the battery 2 and the load 4 is established.

If the ammeter 5 detects the flow of the load current (i.e. “YES” instep S1), in step S2 the load current presently measured is comparedwith the load current previously measured in a previous samplingsequence.

If the presently measured current is higher than or equal to thepreviously measured current (i.e. “YES” in the step S2), that is, if itis determined that the present current flowing to the load 4 has beenincreased or it is constant, the first temperature estimation device 61starts a timer to record the transit time and estimates the temperatureof the wire 7 using the formula (1) (step 4). This estimation is carriedout based on both the current measured by the ammeter 5 and the transittime counted by the timer. Meanwhile, the ambient temperature T2 is setto be 25° C. as an initial value, for example.

In step S7, the fourth temperature estimation device 64 stores thetemperature T1 estimated in the step S4 in the memory 64 a as theestimated present temperature of the wire 7, Tnow.

In step S8, the temperature determination device 65 compares Tnow storedin the memory 64 a with the predetermined allowable temperature Tth.Then, if Tnow is less than or equal to Tth, the process returns to thestep S1 (i.e. “YES” in the step S8).

According to the above steps, if the current flowing to the load 4 isincreasing or stable, the successive steps (S1, S2, S4, and S7) arerepeated and the temperature of the wire 7 is saturated to temperatureT1 as related in the formula (4) described below, which is obtained fromthe formula (1) by substituting an infinite value in the transit timethereof;

T1=T2+i ² ·r·R  (4).

If the so-called rare-short circuit occurs in the load circuit 1 and thecurrent flowing to the load 4 is increased, the temperature of the wire7 is increased and Tnow will exceed Tth. In this case, the decision inthe step S8 becomes “NO”. Accordingly, the switch control device 66turns off the electrical switch 3 so as to stop supplying the power tothe circuit for protection device.

If the presently measured current of the present sampling is less thanthe previously measured current in the previous sampling (i.e. “NO” inthe step S2), it is determined in step S3 if the previously measuredcurrent is outside of the range of current in the normal operation andthe presently measured current is within the range of current in thenormal operation. If arcing has occurred, the answer becomes “YES” andthe arc-induced rising/increasing temperature estimation device 63estimates the arc-induced rising temperature, based on the formula (3)described above with reference to the arc-related map (step S5). Thatis, the temperature increment Q(i) is determined by applying themeasured current, which flows just before the normal current isrecovered, to the arc-related map.

In the step S7, the present temperature estimation device 64 adds theestimated temperature increment Q(i) resulting from the above process,to the stored temperature of the wire 7 in the memory 64 a, thereby thetemperature of the wire 7 is updated. In other words, the temperaturestored in the memory 64 a in the previous sampling sequence is regardedas the ambient temperature T2, and based on the formula (3) the sum ofT2 plus the temperature increment Q(i) becomes the present temperatureT1 of the wire. Accordingly, this T1 is stored in the memory 64 a as theestimated present temperature Tnow of the wire 7.

Next, if the estimated temperature Tnow is less than or equal to theallowable temperature (i.e. “YES” in step S8), the process returns tothe step S1. On the contrary, if “No” is determined in the step S8, theswitch control device 66 turns off the electrical switch 3 to shut offthe power for the circuit to prevent the load circuit 1 from beingoverheated.

If “NO” is determined in the step S3, that is, if a condition where thepreviously measured current is outside of the range of current in thenormal operation and the presently measured current is within the rangeof current in the normal operation (in other words, the condition wherethe current is steadly decreased without any increase of the current byarcing or the like and “NO” is determined in the step S1 (i.e. nocurrent flowing in the load 4), is not satisfied) the process proceedsto step S6.

In the step S6, the falling/decreasing temperature estimation device 62initially resets and restarts the timer. Further, it estimates thepresent temperature of the wire 7, T1, from the formula (2) bysubstituting the transit time, t, counted by the timer. Here, asdescribed above, the current i used in the formula (2) is defined as acurrent which would be flowing at this temperature which is assumed as asaturated temperature. If the temperature of the wire 7 is saturatedwhen the ammeter 5 does not detect the current or detects that thecurrent is decreased, the current i in the formula (2) is defined as acurrent which is measured just before the ammeter 5 does not detect thecurrent or detects that the current has decreased.

Hereinafter, the current i in the formula (2) will be explained indetail. Now it is assumed that the ambient temperature (as an initialtemperature of the wire 7) is T21. Next, when a constant current i1starts flowing continuously, the temperature of the wire 7 starts toincrease and it is finally saturated. FIG. 4 a shows this behavior ofthe temperature by a curve s1. As seen in this figure, the temperaturegradually increases from T21 and is finally saturated at a temperatureT11.

Next, if the ammeter 5 does not detect the current or detects that thecurrent decreases after the temperature of the wire 7 is alreadysaturated to the temperature T11, a current which saturates thetemperature of the wire 7 to a temperature T11, and which is measuredjust before the ammeter 5 does not detect the current or detect that thecurrent is decreased, that is, the current i1 is used as “i” in theformula (2). In this case, as seen in FIG. 4 b, the temperature of thewire 7 starts gradually decreasing as a curve s2 obtained by verticalflip of the curve s1. Finally, the temperature is fallen asymptoticallyto the ambient temperature T21.

If the ammeter 5 does not detect the current or detects that the currentis decreased when the temperature of the wire 7 is not saturated yet,that is, as shown in FIG. 5 a, if the current is not detected or isdecreased at time t1 before the temperature is saturated to thetemperature T11, the current, which saturates the temperature of thewire 7 to an instant temperature, is set to be “i” in the formula (2).That is, the current i2 which saturates the temperature of the wire 7 tothe instant temperature T12, is estimated (c.f. a curve s3), then thiscurrent i2 is used as “i” in the formula (2).

Accordingly, the heat radiation property follows a curve obtained byvertical flip of the curve s3 and the temperature of the wire 7decreases as a curve s4 shown in FIG. 5 b.

Further, if the falling/decreasing temperature is estimated based on theformula (2) in the step S6, the estimated present temperature Tnow,which is stored in the memory 64 a of the fourth temperature estimationdevice 64, is updated in step 7.

That is, the estimated present temperature of the wire 7 Tnow, which isdetermined by taking into account all of the rising/increasingtemperature when the current is increasing, the arc-inducedrising/increasing temperature when the arcing occurs, and the heatradiation when the load current is zero or is decreased, is stored inthe memory 64 a In the step S8, this estimated present temperature Tnowis compared with the allowable temperature Tth. If Tnow is higher thanor equal to Tth, the electrical switch 3 is turned off to shut off thepower for the load circuit. Consequently, if the rare-short circuitoccurs and the temperature of the wire 7 increases, the power for theload circuit can be immediately shut off.

According to the embodiment of the protection device for the loadcircuit, when the current flowing to the load 4 is increased, itestimates the temperature of the wire 7 based on the formula (1), whenthe current is zero or the current is decreased, it estimates thetemperature of the wire 7 based on the formula (2). When the arcingoccurs, it estimates the arc-induced rising/increasing temperature basedon the formula (3). Consequently, the device comprehensively estimatesthe present temperature of the wire 7 from these temperatures describedabove.

As soon as the estimated present temperature of the wire 7 Tnow achievesthe allowable temperature Tth at which a smoke emission from the wire 7and burnout thereof is induced in the load circuit, the electricalswitch 3 is turned off to protect the load circuit. IN more detail,since the protection device estimates the present temperature of thewire 7 based on the intrinsic properties of the wire and determines ifthe power for the load circuit should be shut off based on the estimatedpresent temperature, it can positively shut off the power to protect theload circuit from emitting smoke from the wire and burnout thereof.Further, the device can prevent unexpected shutoff of the power for theload circuit 1 due to quite low heat generated in the wire 7 by thecurrent flowing to the load circuit 1.

Further, when the current flowing to the load 4 becomes zero or thecurrent starts to decrease, the protection device precisely estimatesthe falling/decreasing temperature due to the heat radiated from thewire because this estimation is carried out based on the formula (2) inwhich the current, which is required to be saturated to the temperaturejust before the current has varied, is used. Consequently, even when therare-short circuit accidentally occurs, the protection device canpositively shut off the power for the load circuit before smoke isemitted from the wire 7 and subsequent burnout thereof.

The protection device for the load circuit by the present invention isexplained by the embodiment as shown in the figures. However, thepresent invention is not limited by the embodiment and eachconfiguration in the present invention may be replaced any one which hassame function. For example, the embodiment described above is used forthe load circuit 1 for bulbs, motors and the like in vehicles, however,it may be used for other load circuits.

1. A protection device for a load circuit including a power supply, aswitch and a load, comprising: a current detection device detecting andmeasuring a current in a conductor connected between the power supplyand the load; a first temperature estimation device that estimates arising temperature of the conductor, when the current is detected by thecurrent detection device; a second temperature estimation device thatestimates a falling temperature of the conductor when the currentdetection device fails to detect the current or detects that the currentis being decreased, a third temperature estimation device that estimatesan arc-induced rising temperature of the conductor that is induced byarcing that occurs just after the current has returned to a normalcurrent; a fourth temperature estimation device that estimates a presenttemperature of the conductor based on the estimated rising, falling, andarc-induced rising temperatures; a temperature determination device thatdetermines whether the present temperature of the conductor estimated bythe fourth temperature estimation device exceeds a predeterminedthreshold temperature; a shutoff control device that shuts off power tothe load circuit when the temperature determination device determinesthat the present temperature of the conductor exceeds the predeterminedthreshold temperature, and wherein the rising temperature is estimatedbase on both the current measured by the current detection device andthermal properties of the conductor; the falling temperature of theconductor is estimated based on the thermal properties of the conductor;and the arc-induced rising temperature is estimated based on the currentjust before the current returns to the normal current.
 2. The protectiondevice according to claim 1, wherein the thermal properties are thermalresistivity R and heat capacity C.
 3. The protection device according toclaim 2, wherein the rising temperature estimated by the firsttemperature estimation device is determined by the following formula;$T_{1} = {T_{2} + {i^{2}{r \cdot R \cdot \left( {1 - ^{{- \frac{1}{C \cdot R}} \cdot t}} \right)}}}$, where T1 is a temperature of the conductor [° C.], T2 is an ambienttemperature [° C.], i is the current [A], r is an electrical resistance[Ω], R is the thermal resistivity of the conductor [° C./W], C is theheat capacity of the conductor [J/° C. or W·sec/° C.], and t is transittime [sec].
 4. The protection device according to claim 2, wherein thefalling temperature estimated by the second temperature estimationdevice is determined by the following formula;$T_{1} = {T_{2} + {i^{2}{r \cdot R \cdot ^{{- \frac{1}{C \cdot R}} \cdot t}}}}$, where T1 is a temperature of the conductor [° C.], T2 is an ambienttemperature [° C.], i is the current [A] required to saturate thetemperature of the conductor at which the current is not detected or thecurrent decreases, r is an electrical resistance [Ω], R is the thermalresistivity of the conductor [° C./W], C is the heat capacity of theconductor [J/° C. or W·sec/° C.], t is transit time [sec].